This invention relates to pressure filters and, more particularly, is concerned with pressure filters of the type in which high pressure is applied to a feed mixture of a liquid and a particulate solid by means of an elastic membrane, or diaphragm.
It is known that the higher the pressure which is applied to a feed mixture (comprising a mixture of liquid and particulate solid) in a pressure filter containing a filtering surface, the higher is the proportion of filtrate, i.e. liquid, which is forced through the filtering surface, and the drier is the filter cake, i.e. particulate solid, retained on the filtering surface. In conventional pressure filters the pressure is applied to the feed mixture directly by means of a rotary or reciprocating pump; but if it is desired to operate the pressure filter at a pressure greater than about 300 pounds per square inch it becomes uneconomic to apply pressure directly to the feed because the solid particles in the feed would cause serious wear problems in the moving and stationary parts of any high pressure pump used for the purpose.
There have therefore been developed in recent years a type of pressure filter which uses an elastic membrane, or diaphragm, to transmit pressure from a hydraulic fluid free of solid particles to the feed mixture of liquid and particulate solid to be pressure filtered. The pump used for generating the high pressure therefore comes into contact only with a "clean" liquid and the wear problem is considerably reduced. Generally, a pressure filter incorporating an elastic membrane, or diaphragm, comprises a pressure vessel, the inside of which is divided by the elastic membrane into two compartments, one compartment being for the hydraulic fluid and the other compartment being for the feed mixture (which comprises liquid and particulate solid) and including a filtering surface which is permeable to the liquid but impermeable to the particulate solid of the feed mixture. Such a pressure filter, which is referred to herein as "a membrane pressure filter", can be constructed so that the filtering surface and the surface of the membrane are parallel and either planar or curved; but it is in general more advantageous to provide a substantially cylindrical filtering surface and a coaxial, cylindrical membrane, because this configuration generally provides a more favourable ratio of filtering surface area to volume of the pressure vessel required to contain the filtering surface, its supporting structure and membrane. A membrane pressure filter of this latter type is conveniently referred to as a "tubular membrane pressure filter". Various kinds of tubular membrane pressure filter have been described, see for example British Pat. Nos. 907,485; 1,194,676; 1,240,465 and 1,271,494 and U.S. Pat. No. 3,900,403.
One kind of tubular membrane pressure filter comprises a pair of generally tubular, coaxial, inner and outer assemblies arranged one within the other and adapted to be supported in a generally upright position, an elastic membrane in the form of an impermeable elastic sleeve disposed within and secured at each end to the outer tubular assembly, a filtering surface in the form of a filter element disposed around and supported by the inner tubular assembly, outlet means for the discharge from the interior of the inner tubular assembly of filtrate (i.e. liquid) which has passed through the filter element and through apertures in the inner tubular assembly, and means for displacing the tubular assemblies axially relative to one another between first and second positions, the arrangement being such that in the first position of said tubular assemblies they co-operate with each other to define an annular chamber which is closed and divided into coaxial and non-intercommunicating inner and outer compartments by said impermeable elastic sleeve, the inner compartment having an inlet for a feed mixture (comprising a mixture of liquid and particulate solid to be separated) and the outer compartment having an inlet for hydraulic fluid under pressure, and in the second position of said tubular assemblies said annular chamber is open to enable particulate solid to be discharged from the inner compartment. Hereinafter such a tubular membrane pressure filter will be referred to as "a tubular membrane pressure filter of the kind set forth".
One problem which exists with membrane pressure filters is that, in all cases other than that in which the filtering surface is substantially horizontal, unless the specific gravity of the material constituting the hydraulic fluid is substantially the same as that of the material constituting the feed mixture, the heavier material exerts a higher pressure on the lower parts of the elastic membrane than the lighter material with the result that the elastic membrane is forced to expand into the compartment occupied by the lighter material until the force due to the difference in the hydrostatic pressure acting on the two sides of the membrane is balanced by the tensile force due to the expansion of the membrane. As a general rule, the larger the elastic membrane the greater will be the change in the dimensions of the elastic membrane before the force due to the difference in the hydrostatic pressure balances the tensile force in the elastic membrane. For example, in the case of an elastic membrane in the form of a cylindrical rubber sleeve, as may be used in a tubular membrane pressure filter, the greater the diameter of the sleeve the greater will be the increase in the circumference of the sleeve. The expansion of the elastic membrane into the compartment occupied by the lighter material brings with it the problem that the surface of the elastic membrane is no longer parallel to the filtering surface. Consequently, as the hydrostatic pressure of the hydraulic fluid is increased in order to express the liquid through the pores of the filtering surface one part of the membrane approaches very close to, and may finally touch, the filtering surface thus effectively preventing any further filtration at that part of the filtering surface, while other parts of the elastic membrane remain at a considerable distance from the filtering surface throughout the filtering operation so that a thick layer of filter cake is formed in those parts. The resultant filter cake of varying thickness is difficult to discharge and its formation represents inefficient use of the pressure filter.
One solution to the problem outlined above is to use a hydraulic fluid, the specific gravity of which differs from the specific gravity of the feed mixture by only a small amount (see British Pat. No. 1,240,466). However this solution to the problem suffers from the disadvantage that hydraulic fluids of specific gravity substantially higher than that of water are expensive and often difficult to handle. Also, the specific gravity of the feed mixture may vary widely over a relatively short time interval so that it is impossible to keep the difference between the specific gravities of the materials within the desired limits.